This invention relates generally to gas-insulated electrical apparatus and more particularly to a gas-insulated transmission line having a particle trap which may be field installed.
Gas-insulated transmission lines are being used on an ever increasing scale in recent years due to the desirability of increasing safety, problems in acquiring right-of-way for overhead lines, and high power lines required by growing metropolitan areas and the growing demands for electrical energy. Gas-insulated transmission lines typically comprise a hollow sheath, a conductor disposed within the sheath, a plurality of solid insulating spacers which supports the conductor within the sheath, and a compressed gas such as sulfur hexafluoride disposed within the sheath to electrically insulate the conductor from the sheath. The typical assembly has been fabricated from relatively short sections of hollow cylindrical ducts or tubes in which the conductor and insulators are inserted. This assembly is usually completed in the factory, and the sections are welded or otherwise secured together in the field to form the transmission line. Gas barriers may be provided at intervals along the length of the assembly, and, after evacuation of the line, the insulating gas is forced into the sheath under pressure.
One problem occurring in the use of gas-insulated transmission lines concerns the mobile conducting or semiconducting particle. These particles, which may enter the line during the fabrication of the line during installation in the field, cause problems in that they may lower the dielectric strength of the insulating gas and may initiate flashover and breakdown of the gas as they travel between the outer sheath and the inner conductor. Trump, in U.S. Pat. No. 3,515,939, disclosed a means for deactivating and eliminating the deleterious effects of such conducting particles. In that patent, Trump describes the use of electrodes placed inside the outer sheath to create low field regions which trap and deactivate the particles. Whenever a particle enters the low field region, it cannot acquire enough force to propel it out of the region, and thus it is trapped in the low field region so that it cannot initiate breakdown of the line.
However, the use of the particle trapping region as taught by Trump has not always occurred. Indeed, the majority of the gas-insulated transmission lines installed in this decade have been installed without such particle trapping regions. It is only within the recent past that the magnitude of the problem caused by the conducting particles has been appreciated. As such, there are numerous installations of gas-insulated transmission lines which do not contain such particle traps, or which contain an inadequate number of traps, and it is not unknown for these lines to flashover or breakdown due to the presence of the particles. Therefore, it is desirable that some means be provided for installing particle traps in these previously-installed transmission lines, and these particle traps should be capable of being installed in the field.